Me and my Geeky Hobbies

RF Sniffer

Yet another circuit based on the biased 1N5711 detector topology utilised in the wavemeter and passive VHF receiver projects. The only two differences in this circuit are the lack of a capacitor to resonate the pickup coil and I didn't add the pot to set the quiescent current, using just the 1M2 resistor to the diodes produced a reading just beyond "1" on the scale with the particular MPSA18 I used. Unplugging the coil is the off-switch, but the current consumption is tiny so I usually leave it plugged in.

The pickup coil is slightly exotic, having a jumble-wound LF/MF choke in series with a HF/VHF coil with a varying pitch along its length. The general idea was to try and produce an probe with several resonances in key bands to make the unit more generally usable with the single probe coil. The turns are held in place by dipping the coil in molten wax. Other coils can be attached, it uses an RCA socket like the HF wavemeter and the two units can share coils. I have several for specific purposes. Sensitivity peaks at the self-resonance of the coil and the stray capacitance of the circuit. The hybrid choke probe works fairly well from LF to SHF.

The microwave oven and WiFi base station are easily detected, showing the units SHF response. My various HAM transmitters happily slam the needle across HF to VHF and UHF. It picks up SAW-locked UHF keyfob transmitters, and even stray MF radiation from the LED multiplexing on the air conditioner control panel. LF radiation from 256 kHz contact-less proximity card readers is detectable quite a distance with the hybrid coil.

It is often too sensitive, and a way to vary the sensitivity would be useful. A 5-10k pot in the right place would do the trick if you are building your own. This would increase its usability as a stray-current tracking tool in antenna work. A probe comprising a split ferrite toroid (clamp-on current probe) would also be quite useful but I haven't had the need to build one yet.

Probably the most common use is just to see if an oscillator is making RF. For this purpose it was originally constructed: While experimenting with UHF oscillators around 1.5 GHz, the sniffer was used check the oscillator output, and to detect nulls while playing with Lecher lines.